Respiratory assistance device and a method of controlling said device

ABSTRACT

A method of controlling a flow rate of gases supplied to a patient by a respiratory assistance device includes controlling the supply gases flow rate so as to deliver gases to the patient according to a predetermined gases pressure/flow rate profile for at least a portion of the breathing cycle. A profile may be achieved that provides the patient with a particular benefit or therapy.

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to a method of controlling a respiratoryassistance device and to a respiratory assistance device.

Description of the Related Art

Respiratory assistance devices are often used where a patient or otheruser requires assistance when breathing. This may be to assist inproviding sufficient air to the patient during normal breathing or totreat a particular condition, such as sleep apnoea, for example. Thesemay be used when in hospital, or at home, for example.

Depending on the treatment or patient needs, gases other than air may beused and/or used to supplement air. For example, air may be supplementedwith oxygen to provide a patient with oxygen-enriched air. Otherchemicals, compositions, or medicaments may also be added oralternatively used. Further, the gases may be humidified to provide forimproved patient comfort.

Respiratory assistance devices typically comprise a respiratory gasessource, such as a supply of pressurised gases and/or a gases flowgenerator such as a blower or compressor. The respiratory gases sourcedelivers gases to a patient via an inspiratory conduit connected to apatient interface such as a full face mask, a nasal mask, or a nasalcannula, for example. The gases may be delivered at a pressure greaterthan atmospheric pressure to assist in delivering sufficient gases tothe lungs of the patient and/or to provide respiratory support, forexample, during treatment of chronic obstructive pulmonary disease(COPD).

During normal respiration (unassisted breathing), the pressure and flowrate within the airway of a patient vary. During inspiration, thepressure rapidly decreases from ambient pressure to a maximum negativepressure as the diaphragm moves downward. This causes air to be drawninto the airway with an increasing flow rate, initiating at near zeroflow. As inspiration ends, the pressure returns toward ambient pressureand the flow rate returns to near zero. During subsequent expiration,the pressure rises as the diaphragm moves upward, generating increasingflow in the opposite direction (i.e., out of the airway). As expirationcontinues, the pressure tails off toward ambient pressure and the flowrate returns to near zero for commencement of the next inhalation.

Due to the physiological stress that a patient may experience in anintensive care unit (ICU), the patient can have a higher than usualdemand for oxygen and often require a much greater level of ventilationcompared to a normal, healthy patient. The patient can exert anincreased level of effort of breathing trying to meet this demand and toensure adequate ventilation. This increased level of effort means thatthe patient is expending greater metabolic effort and so producing morecarbon dioxide (CO₂). To remove the CO₂ and keep respiratory gasesconcentrations at a safe level requires even greater ventilation, andtherefore even greater effort. As the patient continually tries toincrease the effort to breathe and is unable to adequately expire theincreasing amounts of CO₂ produced, the patient can experiencerespiratory muscle fatigue and develop hypercapnia.

Elevated levels of CO₂ can also occur in a patient due to an obstructivedisease such as asthma. Excess mucous in the airway andbronchoconstrictions can inhibit breathing and result in the respiratorymuscles being unable to provide sufficient ventilation to meet themetabolic demands of the patient.

Respiratory assistance devices have been proposed which attempt tocontrol their operation to better align with the changing needs of apatient during the breathing cycle.

For example, U.S. Pat. No. 5,148,802 describes an apparatus forproviding alternating high and low positive pressures in the airway of apatient. The pressure levels are coordinated with the spontaneousrespiration of the patient such that the disclosed apparatus provideslower positive pressure during exhalation than during inhalation, todecrease resistance experienced during exhalation. To achieve this, itis necessary for the disclosed apparatus to monitor the breathing cycleof the patient to determine when to switch between the pressure levels.Flow is monitored to determine instantaneous and average flow rates,with inhalation detected if the instantaneous flow rate is greater thanthe average flow rate and exhalation detected if the instantaneous flowrate is less than the average flow rate, and with the pressure levelsbeing adjusted accordingly. While the disclosed apparatus may providefor some improved comfort for a patient, the instant inventors haverecognised that the control provided thereby is not ideal. For example,during inhalation, when there is less resistance to the gases beingdelivered to the lungs since there is no opposing air flow as there isduring exhalation, there may be no need to increase pressure.

The inventors have further recognised that flow resistance in the airwayof a patient is proportional to the flow restriction in the nasalcavities of the patient. Indeed around 50% of the resistance to flow isprovided by the nasal cavities. This nasal resistance is present duringnormal respiration and when the patient is provided with respiratoryassistance. For example, during continuous positive airway pressure(CPAP) treatment, gases are delivered to a patient at an elevatedpressure, but the resistance to flow remains the same. Consequently, theprofile of pressure against gases flow rate during CPAP is the same asthe profile during normal, unassisted respiration, but with the profileshifted on the pressure axis by the amount of pressure elevation. Priorart devices have at least not fully accounted for this relativelysignificant nasal resistance to flow, with flow being monitored based onmeasurements taken along the delivery path to, but prior to reaching,the patient.

It can be desirable to use a nasal high flow or jet delivery system,rather than a face mask based arrangement for some therapies. Further,some patients may prefer such interfaces based on perceived comfortlevels. At present, such devices generally deliver gases at a fixed flowrate.

OBJECT OF THE INVENTION

It is an object of the invention to provide a method of controlling arespiratory assistance device and/or a respiratory assistance devicewhich overcomes or at least ameliorates one or more of the disadvantagesof the prior art.

Alternatively, it is an object to at least provide the public orindustry with a useful choice.

Further objects of the invention will become apparent from the followingdescription.

SUMMARY OF THE INVENTION

In a first aspect, the invention may broadly be said to consist in amethod of controlling a flow rate of respiratory gases supplied to apatient by a respiratory assistance device arranged to be in fluidcommunication with an inspiratory conduit and a patient interface todeliver gases to the patient, and comprising or arranged to be in fluidcommunication with a respiratory gases source, the method comprising:

controlling the flow rate of respiratory gases supplied to the patientaccording to a predetermined gases pressure/flow rate profile in a modedefined for at least a portion of a breathing cycle of the patient,wherein the predetermined gases pressure/flow rate profile is configuredto alter the flow rate at the end of expiration and/or the beginning ofinspiration to increase the clearance of anatomical dead space in thepatient.

Preferably, the controlling comprises:

controlling the flow rate of respiratory gases supplied to the patientaccording to a first predetermined gases pressure/flow rate profile in afirst mode, and

controlling the flow rate of respiratory gases supplied to the patientaccording to a second predetermined gases pressure/flow rate profile ina second mode,

wherein the second predetermined gases pressure/flow rate profile isdifferent to the first predetermined gases pressure/flow rate profile.

Preferably the method comprises an additional step of controlling theflow rate of respiratory gases supplied to the patient according to atleast one further predetermined gases pressure/flow rate profile in atleast one further mode.

Preferably the flow rate of respiratory gases supplied to the patient iscontrolled according to the further profile between inspiration andexpiration.

Preferably the further profile is configured such that the flow rate ofrespiratory gases is supplied to the patient at a pressure determined bya pressure set point.

Preferably the set point corresponds to the positive end expiratorypressure (PEEP).

Preferably the or each profile is configured to achieve at least one ofa plurality of predetermined physiological effects or benefits to thepatient.

For example, the or each profile may be configured to achieve at leastone of:

an increase in inspiratory pressure to reduce inspiratory resistance;

an increase or decrease in expiratory resistance;

an increase in the flow rate of respiratory gases supplied to thepatient at the end of expiration and/or beginning of inspiration toincrease clearance of anatomical dead space independently of inspiratoryand/or expiratory pressure; or

an increase in the flow rate of respiratory gases supplied to thepatient to meet peak inspiratory demand independently of generatedpressure during inspiration and/or expiration to accurately deliveroxygen or other respiratory or anaesthetic gases, aerosols, humidity,etc.

Preferably, during one or more of the modes, the flow rate ofrespiratory gases supplied to the patient is controlled such that therespiratory gases are supplied at a substantially constant pressure forat least a predetermined portion of the breathing cycle.

Preferably, during one or more of the modes, the flow rate ofrespiratory gases supplied to the patient is controlled such that therespiratory gases are supplied at varying pressures for at least apredetermined portion of the breathing cycle.

Preferably the or each profile is configured to provide a supply of gastailored to the patient's respiratory needs at one or more points in thebreathing cycle.

Preferably at least one profile is determined in dependence uponproviding a desired pressure characteristic to the patient.

Preferably at least one profile is determined on the basis of thepressure in the airway of the patient.

Preferably at least one profile is configured such that the pressure ofthe respiratory gases supplied to the patient always equals or exceedsthe pressure required by the patient.

Preferably at least one profile is determined in dependence uponproviding a desired therapeutic effect to the patient.

Preferably the flow rate of respiratory gases supplied to the patient iscontrolled according to more than one of the profiles during inspirationand/or expiration.

Preferably the first profile corresponds to an inspiration mode and/orthe second profile corresponds to an expiration mode.

Preferably the first profile is determined independently of the secondprofile.

Preferably the flow rate of respiratory gases supplied to the patient iscontrolled to supply respiratory gases in pulses at a relatively highflow rate.

Preferably the flow rate of respiratory gases supplied to the patient iscontrolled to supply respiratory gases continuously at a relatively highflow rate.

Preferably the flow rate of respiratory gases supplied to the patient isbetween about 10 and 80 L/min.

Alternatively, the flow rate of respiratory gases supplied to thepatient may be between about 1 and 20 L/min. The lower flow rate may bemore appropriate for a child, for example.

Preferably the flow rate of respiratory gases supplied to the patient iscontrolled in dependence upon a resistance to gases flow provided by thenasal cavities of the patient.

Preferably the controlling comprises dynamically controlling the flowrate of respiratory gases supplied to the patient over time.

Preferably the controlling comprises varying the flow rate ofrespiratory gases supplied to the patient.

Preferably the controlling comprises varying the flow rate ofrespiratory gases supplied to the patient and the varying is performedwhen switching between the modes and/or during one or more of the modes.

Preferably the controlling comprises varying at least one of:

an impedance of a gases flow path between the respiratory gases sourceand a patient interface; and a flow rate of respiratory gases in thegases flow path.

Preferably varying the impedance of the gases flow path is achievedthrough use of a valve or flow restrictor.

Alternatively, or additionally, varying the flow rate of respiratorygases is achieved by controlling the speed of a motor of a gases flowgenerator that comprises, or is arranged to be in fluid communicationwith, the respiratory gases source.

Alternatively, or additionally, the respiratory gases source maycomprise a source of pressurised gas. For example, the respiratory gasessource may comprise or be connected to a cylinder of gases and/or gasesprovided via a compressor. Such gases may provide all or only a portionof the gases supplied to a patient. For example, such a supply may beused to supplement air provided to a patient. For example, air may bedriven toward a patient and enriched along its path with oxygen.

Preferably pressure is determined by calculation from at least one of:

a pressure measured by a pressure sensor;

a flow rate measured by a flow sensor;

a motor speed; and

a known pressure drop across the gases flow path/circuit.

The pressure may be determined via a pressure sensor which may belocated, for example, in a pressure line that is separate from thepatient interface. The pressure sensor may be positioned (and/orpressure may be measured) anywhere between the nasal cavity and therespiratory gases source. The pressure may be determined by calculationfrom, for example, at least one of: motor speed, known pressure dropacross the gases flow path/circuit, and flow rate measured from a flowsensor. Known processes can be used to determine pressure dropsassociated with particular items of equipment.

Preferably the supply gases flow rate is controlled in dependence uponthe resistance to gases flow provided by the nasal cavities of thepatient.

In a second aspect, the invention may broadly be said to consist in amethod of controlling the flow rate of respiratory gases supplied to apatient by a respiratory assistance device arranged to be in fluidcommunication with an inspiratory conduit and a patient interface todeliver gases to the patient, and comprising or arranged to be in fluidcommunication with a respiratory gases source, the method comprising:

controlling the flow rate of respiratory gases supplied to the patientaccording to a predetermined gases pressure/flow rate profile in a modedefined for at least a portion of a breathing cycle of the patient,wherein the predetermined gases pressure/flow rate profile is configuredto control the flow rate of respiratory gases supplied to the patientsuch that respiratory gases are supplied at a substantially constantpressure for a predetermined portion of the breathing cycle.

Preferably said controlling comprises:

controlling the flow rate of respiratory gases supplied to the patientaccording to a first predetermined gases pressure/flow rate profile in afirst mode; and

controlling the flow rate of respiratory gases supplied to the patientaccording to a second predetermined gases pressure/flow rate profile ina second mode,

wherein the second predetermined gases pressure/flow rate profile isdifferent to the first predetermined gases pressure/flow rate profile.

Preferably the first profile is configured to control the flow rate ofrespiratory gases supplied to the patient such that respiratory gasesare supplied at a first substantially constant pressure for apredetermined portion of the inspiratory phase of the breathing cycle;

the second profile is configured to control the flow rate of respiratorygases supplied to the patient such that respiratory gases are suppliedat a second substantially constant pressure for a predetermined portionof the expiratory phase of the breathing cycle; and

the first substantially constant pressure is different to the secondsubstantially constant pressure.

Preferably the controlling comprises dynamically controlling the flowrate of respiratory gases supplied to the patient over time.

Preferably the controlling comprises varying the flow rate ofrespiratory gases supplied to the patient.

Preferably the controlling comprises varying the flow rate ofrespiratory gases supplied to the patient and the varying is performedwhen switching between the modes and/or during one or more of the modes.

Preferably the controlling comprises varying at least one of:

an impedance of a gases flow path between the respiratory gases sourceand a patient interface; and

a flow rate of respiratory gases in the gases flow path.

Preferably varying the impedance of the gases flow path is achievedthrough use of a valve or flow restrictor.

Preferably varying the flow rate of respiratory gases is achieved bycontrolling the speed of a motor of a gases flow generator thatcomprises, or is arranged to be in fluid communication with, therespiratory gases source.

Preferably the respiratory gases source comprises a source ofpressurised gas.

Preferably pressure is determined by calculation from at least one of:

a pressure measured by a pressure sensor;

a flow rate measured by a flow sensor;

a motor speed; and

a known pressure drop across the gases flow path/circuit.

In a third aspect, the invention may broadly be said to consist in arespiratory assistance device arranged to be in fluid communication withan inspiratory conduit and a patient interface to deliver gases to apatient, and comprising or arranged to be in fluid communication with arespiratory gases source, the respiratory assistance device comprising:

a controller operative to control a flow rate of respiratory gasessupplied to the patient according to a predetermined gases pressure/flowrate profile in a mode defined for at least a portion of a breathingcycle of the patient,

wherein the predetermined gases pressure/flow rate profile is configuredto achieve a predetermined physiological effect or benefit to thepatient.

Preferably the respiratory assistance device is operative according toat least one or more of:

a first mode in which the controller varies the flow rate of respiratorygases supplied to the patient according to a first predetermined gasespressure/flow rate profile; and

a second mode in which the controller varies the flow rate ofrespiratory gases supplied to the patient according to a secondpredetermined gases pressure/flow rate profile; and

wherein the second predetermined gases pressure/flow rate profile isdifferent to the first predetermined gases pressure/flow rate profile.

Preferably at least one of the profiles is configured to increase theflow rate at the end of expiration and/or the beginning of inspirationto increase clearance of anatomical dead space in the patient.

Preferably at least one of the profiles is configured to control theflow rate of respiratory gases supplied to the patient such thatrespiratory gases are supplied at a substantially constant pressure fora predetermined portion of the breathing cycle.

Other features of the respiratory assistance device of the third aspectare analogous to the steps recited in relation to the methods of thefirst and second aspects.

In a fourth aspect, the invention may broadly be said to consist in amethod of adjusting a respiratory assistance device arranged to be influid communication with an inspiratory conduit and a patient interfaceto deliver gases to a patient, and comprising or arranged to be in fluidcommunication with a respiratory gases source, the method comprising:

measuring one or more of the positive end expiratory pressure,inspiratory pressure, or expiratory pressure of the patient, the gasespressure in a gases flow path between the respiratory gases source andthe patient airway during inspiration, or the gases pressure in thegases flow path during expiration; and

determining, and/or adjusting, a first predetermined gases pressure/flowrate profile and/or a second predetermined gases pressure/flow rateprofile for controlling the flow rate of respiratory gases supplied tothe patient in dependence upon the or at least one said measuredpressure.

The supply gases flow rate may therefore be controlled and/or adjustedin dependence upon the measurements made to achieve the desired gasespressure/flow rate profile.

Preferably the flow rate of respiratory gases supplied to the patient iscontrolled and/or adjusted in dependence upon the measurements made toachieve the or each profile.

Preferably the method further comprises acquiring an identification of atype of conduit and/or patient interface. Different profiles may berequired dependent on the equipment used for a particular application(e.g., different types of conduits or patient interfaces can affect flowand/or pressure differently, for example, in terms of resistance toflow).

Preferably the identification is acquired via a user input or via anidentification tag associated with the equipment or via an electricalresistance associated with the equipment.

Preferably the identification may include data indicative of at leastone of: patient name, date, gas flow rate, or gas pressure.

Preferably the method further comprises a step to measure PEEP and/or IPand/or EP and/or inspiration gas pressure and/or expiration gas pressureat different gas flow rates.

Preferably the method further comprises a step of providing anindication of a property or characteristic of the device or themeasurements.

Preferably the method further comprises a step of adjusting the flowrate of respiratory gases supplied to the patient by adjusting the motorspeed of a fan motor comprising, or arranged to be in fluidcommunication with, the respiratory gases source. The motor may be anelectronically commutated (EC) motor arranged to receive a directcurrent (DC) supply current, to invert the DC supply to an alternatingcurrent (AC) output, and to control motor speed via control of theinverter. Such a motor has variable speed, and is able to change motorspeed relatively quickly.

In a fifth aspect, the invention may broadly be said to consist in amethod of controlling the flow rate of gas supplied to a patient by arespiratory assistance device, the method comprising controlling thesupply gas flow rate so as to deliver gas to the patient according to apredetermined gas pressure-flow rate profile for at least a portion ofthe breathing cycle.

The method of the fourth aspect may be used in association with therespiratory assistance device of the third aspect of the invention, oras a precursor to, or during, administration of patient therapyaccording to the method of the first, second, or fifth aspects of theinvention.

Further aspects of the invention, which should be considered in all itsnovel aspects, will become apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

A number of embodiments of the invention will now be described by way ofexample with reference to the drawings in which:

FIG. 1 is an overview of a respiratory assistance device in accordancewith the invention;

FIG. 2 is a schematic of a first respiratory assistance device inaccordance with the invention;

FIG. 3 is a schematic of a second respiratory assistance device inaccordance with the invention;

FIGS. 4A and 4B are graphs relating airway pressure to patient flow andtime, respectively, for unassisted breathing and an example variablenasal high flow (VNHF) method and respiratory assistance device inaccordance with the invention;

FIGS. 5A-5C are graphs showing further example gases pressure/flow rateprofiles used with a method and a respiratory assistance device inaccordance with the invention; and

FIGS. 6A and 6B demonstrate varying the flow rate and pressure for theclearance of anatomical dead space according to embodiments of a methodand a respiratory assistance device of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Throughout the description, like reference numerals will be used torefer to like features in different embodiments.

Referring to the figures, an embodiment of a respiratory assistancedevice 1 comprises a respiratory gases source 3 and is arranged to be influid communication with an inspiratory conduit 5 and a patientinterface 7 to deliver respiratory gases to a patient 8. In an alternateembodiment (not shown), the respiratory gases source 3 is separate from,and arranged to be in fluid communication with, the respiratoryassistance device 1. The respiratory assistance device 1 furthercomprises a controller 9 that is operative to control the respiratoryassistance device 1. In some embodiments, the respiratory assistancedevice 1 further comprises a humidifier 10 to heat and humidify therespiratory gases delivered to the patient 8.

The respiratory gases source 3 may incorporate, or be connected to, ablower comprising a motor driving a fan to supply respiratory gases tothe patient 8. The inspiratory conduit 5 may comprise any suitable typeof tubing or the like to form a gases flow path between the respiratorygases source 3 and the patient 8 via the patient interface 7.

The patient interface 7 in this example comprises a nasal cannula. Oneform of nasal cannula typically comprises tubing that extends around theears to the nose of the patient, with an inlet/outlet duct or prongbeing provided at or in each nare and in fluid communication with thetubing. Another form of nasal cannula typically comprises similarinlet/outlet ducts or prongs in fluid communication with tubing held inposition by headgear, such as one or more headstraps. In some cases thetubing may be provided with an exhaust duct or vent. It will beappreciated that any other form of patient interface may alternativelybe used, including a full or partial face mask for example and/or ahybrid thereof.

The controller 9 may comprise an electronic controller, which may bemicroprocessor-based, for example. The controller 9 is operative tocontrol the respiratory assistance device 1 according to a first mode inwhich the supply gases flow rate from the respiratory gases source 3 iscontrolled according to a first predetermined pressure/flow rateprofile; and preferably to further control the respiratory assistancedevice 1 according to a second mode in which the supply gases flow ratefrom the respiratory gases source 3 is controlled according to a secondpredetermined pressure/flow rate profile; the second pressure/flow rateprofile being different to the first pressure/flow rate profile.

The resulting gases flow rate in the gases flow path may be dynamicallyvaried and/or adjusted to achieve a desired pressure/flow rate profile,that is, to achieve a desired gases flow rate and a desired pressure inthe gases flow path over a period of time, and preferably over theduration of the breathing cycle. By achieving such a profile, thedelivery of gases to the patient 8 can be controlled in such a way as toprovide certain benefits, advantages or therapies to the patient 8,and/or to avoid or ameliorate certain disadvantages or problems. Thecontrol can also be such that the profile is achieved for all, or onlycertain portions of the inspiratory and expiratory phases of thebreathing cycle. Different profiles can be achieved for the inspiratoryand expiratory phases of the breathing cycle.

In one example, the respiratory gases source 3 comprises a high speedmotor, such as an electronically commutated (EC) or direct current (DC)brushless motor which drives a fan of a blower and is controlled by thecontroller 9. A flow sensor 13 is provided in the gases delivery pathbetween the respiratory gases source 3 and the patient 8. The controller9 comprises a memory, or is connected to an external memory, on which arelationship between motor speed and gases pressure within the gasesflow path, is stored. This relationship may have been determined vialaboratory testing or in situ testing of the respiratory assistancedevice 1, as explained further below.

The controller 9 then controls the respiratory assistance device 1, andthus the respiratory gases source 3, according to a first mode in whicha first predetermined gases pressure/flow rate profile is achieved. Thefirst profile may be based on a target inspiratory gases flow ratepredetermined either via a suitable algorithm provided on the controller9, or via an operator input. The controller 9 controls the motor speedto achieve the target gases flow rate, on the basis of the predeterminedrelationship between motor speed and gases pressure. The controller 9 isoperative to continuously monitor and control the motor speed to achievethe first gases pressure/flow rate profile. In other embodiments, thegases flow rate may be controlled by the use of a controllable valve orflow restrictor, either alone or in combination with a variable-speedblower motor.

The gases pressure/flow rate profile may be based on a constant valuegases flow rate, for example, through the inspiratory phase of thebreathing cycle. After the peak in inspiration, the gases flow rate willstart to fall and so the controller 9 will reduce the motor speed inrelatively small increments until the gases flow rate starts to increaseagain, immediately after the peak in expiration. At this point, thecontroller 9 may again increase the motor speed such that the gases flowrate at least equals or exceeds the requirements of the patient 8.

In another example, a pressure sensor 15 is provided at or inside thepatient interface 7 to measure the pressure in the airway of the patient8, or at least to measure the pressure in the gases flow path as closeto the airway as possible. The controller 9 receives a signal from thepressure sensor 15 and processes this to control the motor speed toachieve a predetermined pressure level. For example, it may be desirableto maintain a constant pressure level for all or part of the inspiratoryand/or expiratory phases of the breathing cycle, or to achieve differentpressure levels during inspiration and expiration.

In another example, the pressure drop across the gases flow path may bemeasured and input to the controller 9. In this circumstance, thepressure sensor 15 may be mounted on the high pressure side of therespiratory gases source 3, the relationship between pressure drop andgases flow rate along the gases flow path being stored on or referencedby the controller such as via a lookup table or database. The controller9 then controls the motor speed, and hence the supply gases flow rate,with reference to the known pressure drop and additional pressuremeasurements made by the pressure sensor 15, to achieve a predeterminedpressure/flow rate profile. This profile may be configured such that thepressure at the patient interface 7 is at a known flat level.

In a further example, the controller 9 is arranged to measure and storethe pressure change within the gases flow path with upper airway andtidal volume, over a number of breathing cycles for a fixed motor speed.This is done at, or as close to, the desired target flow rate aspossible. The controller 9 can subsequently control the motor speed suchthat the pressure change is near zero for example.

It is envisaged that the respiratory assistance device 1 could be usedwith a patient interface 7 comprising a semi-sealed cannula. Asemi-sealed cannula may be arranged to permit exhaust of the gases flowwhen a predetermined pressure is exceeded, for example during expiratoryflow when the flow from the respiratory assistance device 1 is combinedwith the flow from the lungs of the patient 8. Such a cannula istypically arranged not to allow exhaust of gases flow below apredetermined pressure, for example during inspiration. This would serveto exaggerate the flow difference between inspiration and expirationwhich could subsequently be controlled by varying the motor speed duringthe inspiratory and expiratory phases of the breathing cycle using anyof the methods described above.

Variable nasal high flow (VNHF) can be achieved by controlling the motorspeed to vary the flow rate. Alternatively VNHF can be achieved byvarying the impedance within the flow path of the system, for examplevia application of variable (relatively low) back pressure.

Referring to FIGS. 4A and 4B, the relationships between airway pressureand patient flow rate (FIG. 4A) and time (FIG. 4B) are shown for bothnormal respiration (unassisted breathing) and respiration during VNHFtherapy. The respective pressure/flow rate profiles are substantially asshown in FIG. 4A. In this example, the VNHF supply gases flow rate fromthe source of pressurized gases is controlled to remain substantiallyconstant, for example at 15 litres per minute (L/min) throughoutinspiration and expiration such that the gases pressure increasesgradually from peak inspiration to peak expiration. This providespressure relief during expiration. FIG. 4B shows a single completebreathing cycle (i.e., including an expiratory phase and an inspiratoryphase). In both of the figures the peak expiratory pressure 40, positiveend expiratory pressure (PEEP) 41, and peak inspiratory pressure 42 canbe observed.

Referring to FIG. 5A, in a further example, the supply gases flow ratefrom the respiratory gases source 3 is controlled to decrease betweenpeak inspiration and peak expiration such that the gases pressureremains substantially constant at a medium value of about 2 centimetresof water (cmH₂O). VNHF is different from non-invasive ventilation (NW,for example using a full face mask), where inspiratory pressure (IP) ishigher than expiratory pressure (EP). With VNHF, it would be highlyunlikely to achieve a high IP. In this example, positive airway pressureis maintained at 2 cmH₂O throughout the duration of the respiratorycycle with VNHF.

Referring to FIG. 5B, another example gases pressure/flow rate profileis shown for the inspiratory and expiratory phases of the breathingcycle. Thus the respiratory assistance device 1 is operative accordingto a first profile during inspiration (where the patient flow is shownas negative), and a second profile during expiration (where the patientflow is shown as positive). In this example, the gases flow rate fromthe respiratory gases source 3 is controlled such that the gasespressure during inspiration is substantially constant and relativelylow, held at about 1 cmH₂O. Thus the supply gases flow rate from therespiratory gases source 3 is controlled to reduce during inspiration.Between inspiration and expiration, the supply gases flow rate from therespiratory gases source 3 is controlled to rise substantiallyvertically, such that the gases pressure also rises substantiallyvertically, with the patient gases flow substantially zero. Duringexpiration, the supply gases flow rate from the respiratory gases source3 is controlled such that the pressure remains substantially constantand relatively high, held at about 5 cmH₂O. Thus the supply gases flowrate from the respiratory gases source 3 is again controlled to reduceduring expiration. A decrease of the supply gases flow rate duringexpiration can keep expiratory pressure steady or prevent a highincrease. It can be particularly useful in a hybrid mask where leakage(biased flow) is not high, and flow during inspiration should be higherthan during expiration. Here positive airway pressure is maintained atalternating values of 1 cmH₂O and 5 cmH₂O throughout the duration of therespiratory cycle with VNHF.

Referring to FIG. 5C, in another example, the supply gases flow ratefrom the respiratory gases source 3 is controlled to remainsubstantially constant during inspiration such that the gases pressurerises gradually from 1 cmH₂O toward a relatively high peak of around 5cmH₂O. Slightly before expiration, when the inspiratory gases flow ratefrom the patient 8 is approaching zero, the supply gases flow rate fromthe respiratory gases source 3 is controlled to decrease such that thegases pressure during expiration remains substantially constant andrelatively high, at about 5 cmH₂O.

The patient 8 may experience physiological stress in an intensive careunit (ICU) that makes it more difficult to breathe because the patient 8is unable to adequately expire the increasing amounts of CO₂ produced inthe airway of the patient 8. The patient 8 may also have an obstructivedisease, such as asthma, that can inhibit breathing. In either case, thepatient 8 may benefit greatly from the clearance of anatomical deadspace, also referred to as CO₂ flushing, as a way to help regulate CO₂levels.

In one approach, CO₂ may be removed from the airway of the patient 8during the expiratory pause, immediately before the beginning ofinspiration. At this point, the gases in the airway make up the initialvolume of the following breath, so flushing the airways at this pointreduces rebreathed CO₂. However, high flow rates during expiration canresult in uncomfortable pressure and noise. In particular, flushing theCO₂ at the end of expiration may cause incomplete expiration. In anotherapproach, therefore, CO₂ may be removed from the airway of the patient 8at the beginning of inspiration, or at the end of expiration and thebeginning of inspiration.

Referring to FIGS. 6A and 6B, an example variation of the flow rate andpressure to increase clearance of dead space is shown. The respiratoryassistance device 1 is operative according to a first profile duringinspiration to deliver the illustrated constant inspiratory flow ratethat meets the peak inspiratory demand of the patient 8, a secondprofile during expiration to avoid unnecessary additional flow andtherefore pressure, and a further profile during a “flushing period”substantially between expiration and inspiration. In this example, thefirst profile delivers a flow rate of approximately 35 L/min and thesecond profile delivers a flow rate of approximately 2 L/min. In otherembodiments, the flushing period may alternatively form part of thefirst or second profiles.

The supply gases flow rate from the respiratory gases source 3 iscontrolled to rapidly decrease at the start of expiration to about 2L/min to aid expiratory effort and reduce the potentially uncomfortablesharp increase in pressure and noise that can be felt by the patient 8at this point. In the flushing period, about 1 second before the startof inspiration, the supply gases flow rate from the respiratory gasessource 3 is controlled to increase, reaching for example about 60 L/minafter about 0.5 seconds, and then to be held at about 60 L/min for thelast about 0.5 seconds of expiration. At the start of the nextinspiration, the supply gases flow rate from the respiratory gasessource 3 is controlled to drop back to about 35 L/min.

This method of varying the flow rate targets flushing at the expiratorypause while reducing or minimising the effect of a rapid pressureincrease felt by the patient 8 at the start of expiration. The timing ofthe flushing and the duration of the flushing period may be determinedby measuring and taking the average length of expiration over a numberof the previous breathing cycles, and may also be customised to thepatient 8 in some embodiments. In some embodiments, the flow rate may beincreased in the flushing period to levels lower than 60 L/min, but istypically increased to at least 25 L/min for adults.

The changes in flow rate within or between each profile may besubstantially instantaneous, or there may be a ramp up to and/or rampdown from the peak flow rate. These ramps may have the same or unequalgradients. There may also be multiple flushing phases within a flushingperiod. For example, there may be a ramp up to one flow rate level thatis held for a period of time and then the flow rate may change to asecond level for a second period of time. The flushing period may followvarious waveform shapes. In the illustrated example, the flow rate isincreased to a predetermined level and then held for a period of time.In alternative embodiments, the flow rate during flushing may resemble atriangular or sinusoidal wave, for example.

Any other desired gases pressure/flow rate profile can be achieved asrequired to provide the desired benefit, treatment or therapy to thepatient 8. Any number of profiles can be used, within the breathingcycle or within any part of it.

The respiratory assistance device 1 may be used initially in a test orcalibration mode in which pressure measurements are made undercontrolled conditions by measuring one or more of the PEEP, IP, or EP ofthe patient 8, the gases pressure in a gases flow path between therespiratory gases source 3 and the airway of the patient 8 duringinspiration, or the gases pressure in the gases flow path duringexpiration. At least one of these measurements may be used tosubsequently determine, and/or adjust, the first pressure/flow rateprofile in dependence upon at least one said measured pressure. Thecontroller 9, when in the testing or calibration mode, may refer to alook-up table or database of pressure and/or flow rate characteristicsof the inspiratory conduit 5 and/or patient interface 7 to be used withthe respiratory assistance device 1. For example, a cannula may havedifferent pressure and/or flow rate characteristics from a full facemask, and the controller 9 may refer to these characteristics to controlthe respiratory device 1 to achieve the desired pressure/flow rateprofile accordingly.

The supply gases flow rate may therefore be controlled and/or adjustedin dependence upon the measurements made to achieve the desired gasespressure/flow rate profile. Particular pressure measurements may beassociated with further particular modes of operation and/or theparticular equipment forming the inspiratory conduit 5 and/or thepatient interface 7.

Unless the context clearly requires otherwise, throughout thedescription, the words “comprise”, “comprising”, and the like, are to beconstrued in an inclusive sense as opposed to an exclusive or exhaustivesense, that is to say, in the sense of “including, but not limited to”.

Although this invention has been described by way of example and withreference to possible embodiments thereof, it is to be understood thatmodifications or improvements may be made thereto without departing fromthe scope of the invention. The invention may also be said broadly toconsist in the parts, elements and features referred to or indicated inthe specification of the application, individually or collectively, inany or all combinations of two or more of said parts, elements orfeatures. Furthermore, where reference has been made to specificcomponents or integers of the invention having known equivalents, thensuch equivalents are herein incorporated as if individually set forth.

Any discussion of the prior art throughout the specification should inno way be considered as an admission that such prior art is widely knownor forms part of common general knowledge in the field.

What is claimed is:
 1. A method of controlling a flow rate ofrespiratory gases supplied to a patient, the method comprising:controlling the flow rate of respiratory gases supplied to the patientaccording to a predetermined gases flow rate profile in a mode definedfor at least a portion of a breathing cycle of the patient, wherein thepredetermined gases flow rate profile is configured to control the flowrate of respiratory gases supplied to the patient such that respiratorygases are supplied at a substantially constant pressure over the entireexpiratory phase of the breathing cycle wherein the flow rate isvariable, and wherein the respiratory gases are supplied to the patientusing a nasal cannula.
 2. The method of claim 1, wherein controlling theflow rate of respiratory gases comprises: controlling the flow rate ofrespiratory gases supplied to the patient according to a firstpredetermined gases pressure/flow rate profile in a first mode; andcontrolling the flow rate of respiratory gases supplied to the patientaccording to a second predetermined gases pressure/flow rate profile ina second mode, wherein the second predetermined gases pressure/flow rateprofile is different to the first predetermined gases pressure/flow rateprofile.
 3. The method of claim 2, wherein: the first profile isconfigured to control the flow rate of respiratory gases supplied to thepatient such that respiratory gases are supplied at a firstsubstantially constant pressure for a predetermined portion of aninspiratory phase of the breathing cycle; the second profile isconfigured to control the flow rate of respiratory gases supplied to thepatient such that respiratory gases are supplied at a secondsubstantially constant pressure for a predetermined portion of anexpiratory phase of the breathing cycle; and the first substantiallyconstant pressure is different to the second substantially constantpressure.
 4. The method of claim 1, wherein controlling the flow rate ofrespiratory gases comprises dynamically controlling the flow rate ofrespiratory gases supplied to the patient over time.
 5. The method ofclaim 1, wherein controlling the flow rate of respiratory gasescomprises varying the flow rate of respiratory gases supplied to thepatient.
 6. The method of claim 1, wherein controlling the flow rate ofrespiratory gases comprises varying the flow rate of respiratory gasessupplied to the patient, wherein the varying is performed when switchingbetween the modes and/or during one or more of the modes.
 7. The methodof claim 1, wherein controlling the flow rate of respiratory gasescomprises varying an impedance of a gases flow path between arespiratory gases source and a patient interface or varying a flow rateof respiratory gases.
 8. The method of claim 7, wherein varying theimpedance of the gases flow path is achieved through use of a valve or aflow restrictor.
 9. The method of claim 7, wherein varying the flow rateof respiratory gases is achieved by controlling the speed of a motor ofa gases flow generator.
 10. The method of claim 1, wherein pressure isdetermined by calculation from at least one of: pressure measured by apressure sensor, a flow rate measured by a flow sensor, a motor speed,and a known pressure drop across the gases flow path/circuit.
 11. Arespiratory assistance device comprising: a flow generator fordelivering a respiratory gases to a patient; a non-sealing interface influid communication with the flow generator and providing therespiratory gases to the patient; a controller configured to control aflow rate of respiratory gases supplied to a patient according to apredetermined gases flow rate profile in a mode defined for at least aportion of a breathing cycle of the patient, wherein the predeterminedgases flow rate profile is configured to control the flow rate ofrespiratory gases supplied to the patient such that respiratory gasesare supplied at a substantially constant pressure over the entireexpiratory phase of the breathing cycle wherein the flow rate isvariable.
 12. The respiratory assistance device of claim 11, wherein thecontroller is configured to control the flow rate of respiratory gasessupplied to the patient according to a first predetermined gasespressure/flow rate profile in a first mode, wherein the controller isconfigured to control the flow rate of respiratory gases supplied to thepatient according to a second predetermined gases pressure/flow rateprofile in a second mode, wherein the second predetermined gasespressure/flow rate profile is different to the first predetermined gasespressure/flow rate profile.
 13. The respiratory assistance device ofclaim 12, wherein the first profile is configured to control the flowrate of respiratory gases supplied to the patient such that respiratorygases are supplied at a first substantially constant pressure for apredetermined portion of an inspiratory phase of the breathing cycle,wherein the second profile is configured to control the flow rate ofrespiratory gases supplied to the patient such that respiratory gasesare supplied at a second substantially constant pressure for apredetermined portion of an expiratory phase of the breathing cycle,wherein the first substantially constant pressure is different to thesecond substantially constant pressure.
 14. The respiratory assistancedevice of claim 11, wherein the controller is configured to dynamicallycontrol the flow rate of respiratory gases supplied to the patient overtime.
 15. The respiratory assistance device of claim 11, wherein thecontroller is configured to vary the flow rate of respiratory gasessupplied to the patient.
 16. The respiratory assistance device of claim11, wherein the controller is configured to vary the flow rate ofrespiratory gases supplied to the patient, wherein the varying isperformed when switching between the modes and/or during one or more ofthe modes.
 17. The respiratory assistance device of claim 11, whereinthe controller is configured to vary an impedance of a gases flow pathbetween a respiratory gases source and a patient interface or whereinthe controller is configured to vary a flow rate of respiratory gases.18. The respiratory assistance device of claim 17, wherein varying theimpedance of the gases flow path is achieved through use of a valve or aflow restrictor.
 19. The respiratory assistance device of claim 17,wherein varying the flow rate of respiratory gases is achieved bycontrolling the speed of a motor of a gases flow generator.
 20. Therespiratory assistance device of claim 11, wherein pressure isdetermined by calculation from at least one of: pressure measured by apressure sensor, a flow rate measured by a flow sensor, a motor speed,and a known pressure drop across the gases flow path/circuit.
 21. Themethod of claim 1, wherein the respiratory gases are supplied to thepatient using a high flow respiratory system comprising a flow generatorfor delivering a high flow of respiratory gases to the patient and thenasal cannula is in fluid communication with the flow generator.
 22. Themethod of claim 21, wherein the flow rate of respiratory gases suppliedto the patient is between 10 L/min and 80 L/min.